1. Technical Field
The disclosure relates to zero intermediate frequency (ZIF) based radio solutions and, more particularly, to a method and apparatus for removing DC offset in a ZIF based radio when digital frequency correction is utilized.
2. Related Art
ZIF based radio receivers receive radio frequency (RF) signals that are applied to quadrature mixers which multiply the signal at a frequency generated by a local oscillator (LO). The in-phase signal or I-channel and the quadrature signal or Q-channel are generated by the LO. The LO input down converts the RF signals modulated on a common carrier frequency. The mixers yield outputs that contain both sum frequency components and difference frequency components around zero frequency. DC filters eliminate the sum frequency components and accept the difference frequency components. The zero frequency components are then amplified by low level frequency amplifying stages. The ZIF receiver eliminates interim conversion to an intermediate frequency by converting the incoming signal directly to baseband in a single operation.
ZIF based radio receivers, however, generate DC offset that can be much greater than the desired signal. Low level frequency amplifiers can be saturated by large DC offsets before the desired signal is amplified. Therefore, wireless communication systems that utilize ZIF radios have to overcome large DC offsets in the baseband signal. The DC offset should be removed before the frequency correction of the received baseband signal can be performed, or the signal will turn into a tone at frequency correction with the AFC loop correcting the DC offset. In a GSM receiver, since the bandwidth is relatively small, any significant frequency offset must be performed prior to the final channel filtering. Therefore, what is needed is an efficient way to remove DC offset in ZIF based radio receivers.